Telecommunication standards such as the IEEE 802.16 Standard for broadband wireless access (BWA) has imposed clear and often challenging requirements on the radio portion of the communication systems. The most recent revisions have increased channel bandwidth through scalable addition of sub-carriers for the orthogonal frequency division modulation (OFDM) versions of the physical layer (PHY). This increase in bandwidth comes with little relief for the transceiver fidelity requirements. With the introduction of mobility to the standard set, greater power control accuracy has been demanded of the system. All of this comes with no relief in the worldwide regulatory requirements on emissions and receiver robustness.
Counter to these changes in the standards are the never-ending market demands for more functionality and lower cost. The cost pressures have driven many radio frequency integrated circuit (RFIC) system-on-chip (SOC) manufacturers to turn to more space and cost effective architectures such as direct conversion. This architecture comes with inherent performance shortcomings, which need to be mitigated through various on and off-line strategies.
One class of such strategies involves transceiver calibration. To meet the system specifications defined in the telecommunication standards such as the IEEE 802.16 standard, a radio frequency (RF) system needs to be calibrated for gain and phase imbalance. This is critical for communication systems having high signal quality requirements such as a transmitter for which a 30-dB error vector magnitude (EVM) specification applies. EVM is a method for assessing the quality of digitally modulated telecommunication signals. The major contribution to EVM performance is in the transmitter with the power amplifier operating at the maximum output power. The effects of gain and phase imbalance on EVM need to be minimized to have a very small contribution in this case.